[mythtv-users] Semi-OT - Dual TV antennas

John Finlay finlay at moeraki.com
Fri Aug 21 21:38:07 UTC 2009


Brian Wood wrote:
> On Friday 21 August 2009 13:14:40 Colin McGregor wrote:
>   
>> Up until now I have been using one Winegard GS-2200 antenna on my roof
>> to provide an over the air TV signal for my MythTV box. This has
>> worked okay, but I am wondering ... as I have a second identical
>> GS-2200:
>>
>> - Would there be an advantage to setting up a 2nd GS-2200 antenna?
>> - The GS-2200 is an amplified antenna (using a small power brick in
>> the attic), does that brick provide enough power to support 2
>> antennas?
>>     
>
> Are you planning to combine the two antennas? Or are you planning to use them 
> as seperate sources (perhaps one antenna to each of two cards)?
>
> The problem with combining the two antennas is phase - unless the signals are 
> perfectly in phase you will get some cancellation and other problems, which 
> will vary by channel.
>
> Combining the antennas is likely to cause more trouble than it cures, unless 
> the spacing is perfect, and you use a proper phasing harness, and even then 
> it would only work properly on a single channel.
>   
> There are other things you can do with a second antenna, if you have a severe 
> reflection you could use point the second antenna off-axis to point at the 
> major reflection, invert the phase and use that signal to help cancel out the 
> reflection.
>
> As for your question, I'd have to know what the power draw of the amplifiers 
> was, and the rating of the power supply, but I'd guess that you probably 
> could power two amps from the power brick, those amplifiers donot draw much 
> power.
>
>   
This seems overly pessimistic given that many commercial antennas are 
made up of ganged antennas. For example the CM4228 is basically 2 
CM4221s ganged together and so on. Of course those have been carefully 
designed and built to ensure the additional gain is realized. Generally 
the advice in the hdtvprimer.com website referenced bay another poster 
is a good starting point. In my experience ganging antennas is the most 
difficult to get right and easy to get wrong. I currently use a pair of 
ganged CM4228s with a gain of about 2dB over a single CM4228 but I have 
fairly optimal conditions with minimal ground reflections. In general 
equal lengths of coax (required) and close proximity of the antennas 
makes for best probability of success but significant amounts of 
experimentation are required.

It's much easier to combine antennas for different frequency bands using 
diplexers. I use a diplexer to combine a Hi-VHF antenna with my CM4228s 
and then combine that signal with another diplexer to add some Lo-VHF 
analog signals from a fourth antenna.

The jointenna may allow you to add a single channel from another 
direction as a final option for receiving a particularly difficult channel.

It's a hobby.

John



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